The invention relates to plastic encapsulation of electronic devices, and more specifically, to injection molding an encapsulation for an electronic device directly onto a substrate such as a printed circuit board.
It is well known that electronic devices are sensitive to the environment and that exposure to normal atmospheric conditions may degrade or ruin them entirely. Accordingly, it is the current practice to protect electronic devices from environmental/atmospheric exposure by sealing them within a protective enclosure, commonly made of a non-electrically conducting material such as a plastic resin, with an interfacing means, such as pins, to allow connection of the devices to a larger electronic circuit or other devices. Simple devices such as resistors, capacitors, diodes and the like, as well as more complex semiconductor devices, or chips, are commonly packaged in this manner.
It is common practice to interface such an encapsulated device with other devices mounted on a supporting substrate by, for example, inserting its interface pins into a corresponding socket mounted on the substrate. These other devices are similarly mounted and connected to each other with wires, or with traces in the case where the substrate is a printed circuit board.
This practice of encapsulation suffers from a number of drawbacks. Generally, the equipment and materials necessary to accomplish the encapsulation must be located outside of the clean room environment where the device itself is manufactured, and the encapsulation must therefore be performed as a separate manufacturing step. The encapsulating process is also expensive. Further, the plastic packages themselves, with the required interface means, significantly increase the size of the device, thereby requiring a larger area, or more real estate, for their incorporation in another device or circuit.
Alternatively, it is known that certain electronic devices may be mounted to a substrate such as a printed circuit board, typically with gold wire connections, and encapsulated by a liquid resin that is hand cast over the device on the substrate. This procedure is not preferred because it is expensive, time-consuming, difficult to accurately place the cast material over the device, and provides poor adhesion of the cast material to the substrate. Further, the gold wire connections to the device are very delicate and are easily disconnected during the casting process.
Another prior-art method for encapsulating devices mounted onto a substrate by gold wires is the so-called xe2x80x9ctransfer moldingxe2x80x9d method. Transfer molding is a process by which a thermosetting material is caused to flow into a cavity formed by the cooperation of a mold and a cavity plate. The material enters the cavity through so-called xe2x80x9csidexe2x80x9d gates which are also formed by the space between the cavity plate and the mold. This method is an improvement over the hand-casting encapsulation method because it allows multiple devices to be encapsulated at the same time in one production cycle and it allows for somewhat more accurate placement and size of the resulting encapsulated package.
Transfer molding, however, itself suffers from a number of significant drawbacks which are eliminated by the present invention. Initially, transfer molding techniques of encapsulating electronic devices are limited to use of thermosetting materials which have a low viscosity. Such a material is necessary to prevent damage to the delicate connections of the device to the substrate during the molding process. This same danger requires that the encapsulating material be forced into the mold at low pressure. Use of a low viscosity thermoset results in the need for an expensive mold apparatus which must be constructed with very high tolerances to prevent leakage of the encapsulating material. Even the most expensive molds, however, exhibit some leakage in the area of the gate and device connections which must be removed by additional process steps after molding, thereby increasing cycle times.
Use of a thermoset, which cures by a chemical process, also results in long cycle times, on the order of 5 to 15 minutes, which increases production costs. Thermoset materials themselves are expensive due, in part, to the inability to reuse excess encapsulating material resulting from the molding process after the material has cured.
Use of a low viscosity thermoset at low insertion pressure also results in the need for large side entry gates for the encapsulating material. The large side gates make transfer molding impractical for small devices because the size of the gate limits the size of the cavity. The gating used in transfer molding techniques adds further limitations to the placement and configuration of the devices to be encapsulated because it requires the devices to be near an edge of the substrate to which it is to be bonded. Typical transfer molding applications therefore involve devices which are mounted in a linear arrangement on a substrate with the use of strip-like carriers, or xe2x80x9clead frames,xe2x80x9d for the devices. Transfer molding with thermosets is also not useful with small devices because the thermosetting material requires substantial surface area in contact with the substrate in order to adhere sufficiently to hold the device and encapsulate to the substrate with a chemical or adhesive bond.
On the other hand, use of higher viscosity thermoplastic materials is not practical in transfer molding because it requires higher pressures that may damage the device connections and may result in additional leakage of the encapsulating material. Further it is difficult in a transfer molding apparatus to maintain the high temperatures required to allow a thermoplastic material to properly flow.
Attempts have been made to solve the problems with prior-art encapsulating methods by use of injection molding. Prior art injection molding methods, however, suffered from similar drawbacks. Although higher pressures may be used with injection molding and thus would allow use of thermoplastic materials, the injection process would damage the delicate device connections. In addition, prior art injection molding methods and devices were not useful for small devices because the smaller gating necessitated by smaller cavities had a tendency to clog with the thermoplastic material and this material exhibited poor adhesion resulting in devices being separated from the substrate.
In general, in one aspect, the invention features a method of encapsulating a small electronic device mounted directly on a substrate by providing a three-dimensional formation on the substrate adjacent to the device and injection molding a thermoplastic encapsulating material to cover the device and extend over the three-dimensional formation on the substrate and wherein the encapsulating material mechanically bonds to the three-dimensional formation. In another aspect, the invention features a method of encapsulating a light emitting diode (LED) mounted directly on a substrate by providing a hole through the substrate adjacent to the LED and injection molding a light-transmissive thermoplastic encapsulating material to cover the LED and fill the hole. In another aspect the invention features a method of encapsulating a set of LEDs mounted directly to a printed circuit board (PCB) and arranged to form an alphanumeric display by providing a hole through the PCB adjacent to each of the LEDs, injection molding a light-transmissive thermoplastic around each of the LEDs and wherein each of the LEDs is separately encapsulated in a package that is shaped to focus and reflect light from the LED and is mechanically bonded to the PCB. In a further aspect, the invention features a method of encapsulating a plurality of small electronic devices mounted directly on a substrate in close proximity to one another comprising providing a three-dimensional formation on the substrate adjacent to each device, injection molding a thermoplastic encapsulating material to individually cover each device and wherein the encapsulating material mechanically bonds to the three-dimensional formation on the substrate.
In another aspect the invention features a mold for injection molding a thermoplastic encapsulating material over a small electronic device mounted directly on a substrate, the mold comprising a base member, a top member including an inlet, and a gate plate including a short gate having an input communicating with the inlet and an output communicating with a cavity, and wherein the gate is formed entirely within the gate plate. In another aspect the invention features a gate plate for use in injection molding a thermoplastic encapsulating material over a small electronic device mounted directly on a substrate comprising a cavity formed to enclose the device, and a short gate having an input for receiving an encapsulating material and an output communicating with the cavity, and wherein the gate is formed entirely within the gate plate. In a further aspect, the invention features a gate plate for use in injection molding a light-transmissive thermoplastic encapsulating material over a set of LEDs mounted directly on a substrate comprising a set of cavities arranged in the form of an alphanumeric display, each cavity formed to enclose one LED and extend over its adjacent hole and a set of short conical gates, each having an input for receiving an encapsulating material and an output communicating with a cavity, and wherein the gates are formed entirely within the gate plate.
In another aspect, the invention features an encapsulated electrical device mounted directly on a substrate wherein the device is fully encapsulated by an encapsulating material which is injection molded onto and mechanically bonded to the substrate. In a further aspect, the invention features an alphanumerical display comprising a set of LEDs mounted directly to a PCB with holes through the PCB adjacent to each of the LEDs and arranged to form a display and wherein each of the LEDs is separately encapsulated in a package of light-transmissive material that has been molded onto and mechanically bonded to the PCB.
Preferred embodiments of the invention include one or more of the following features. A three-dimensional feature on the substrate adjacent to the device to be encapsulated where the three-dimensional feature is optionally a hole, a raised member extending above the surface of or a groove extending below the surface of the substrate. A substrate comprising a printed circuit board. An encapsulating material comprised of a thermoplastic resin chosen from among the groups of polycarbonates and acrylics. A method wherein, after injection, the encapsulating material is located on less than the entire surface of the substrate. Encapsulating a light emitting diode in a light-transmissive encapsulating material. Encapsulating on a PCB a light emitting diode using a lozenge shaped hole adjacent to the LED and wherein, after encapsulation, the encapsulating material on the side of the PCB opposite from the light emitting diode is substantially flat and flush with the surface of the printed circuit board. Encapsulating a plurality of LEDs on a PCB in the form of an alphanumeric display including, for example, a seven segment display. Encapsulating a plurality of devices arranged in a non-uniform arrangement on a substrate and wherein one or more of the devices are located away from the edges of the substrate. A gate plate having a substantially conical gate. A gate having a cross sectional area that is reduced from the input to the output to form a region of relatively reduced strength in hardened encapsulating material, whereby the material is caused to break off near to the output of the gate when the mold is opened. A gate having sidewalls that are inclined about 15 degrees over the length of the gate from the input to the output. A gate plate about 0.250 inches thick. A gate plate including a distribution runner connected to the gate. A gate about 0.065 inches long.
A set of cavities arranged in the form of an alphanumeric display. A set of cavities arranged in the form of a seven-segment display. A cavity shaped to form a package of encapsulating material that focuses and reflects light from a light emitting diode out of an adjacent hole in the substrate. A cavity having a base opposite from the output of the gate, which base is substantially triangular with rounded corners and which cavity is substantially rounded above its base. An encapsulated electrical device mounted directly on a substrate wherein the device is fully encapsulated by an encapsulating material. An alphanumerical display comprising an array of light emitting diodes separately encapsulated in a package of light-transmissive thermoplastic material.
Among the advantages of the invention are one or more of the following. The invention eliminates the need for separate, expensive and time-consuming encapsulation of electronic devices. The invention eliminates the need for pin interfaces on circuit boards. The invention reduces the amount of space required on a circuit board for a given electronic device. In one aspect, the invention provides an injection molding method where the encapsulating material hardens quickly by loss of heat, without leakage of material, thereby increasing production speed and capacity. The invention increases production speed by eliminating the need to mill excess encapsulating material after molding. The invention allows for economical reuse and recycling of a thermoplastic encapsulating material. In another aspect, the invention provides apparatus for injection molding that is efficient and economical. In another aspect, the invention provides an injection mold with a thin gate plate that is separate from the larger top and bottom plates of the mold and that may be designed to be constructed of relatively inexpensive material and therefore economically replaced when worn. The invention further provides an injection molding apparatus that allows placement of an encapsulating material in any location and in any configuration on a substrate. In another aspect the invention provides an injection molding apparatus that allows many small devices to be individually encapsulated in close proximity to one another. The invention alleviates the potential problems of wires or traces being torn or disconnected from the device and prevents the device from being separated from the substrate by making use of small devices that may be mounted directly to the substrate without wires and by mechanically bonding the encapsulated material to the substrate. In another aspect, the invention quickly and economically encapsulates a light emitting diode on a printed circuit board using a minimum of real estate. The invention further provides an alphanumeric display economically constructed on a printed circuit board by injection molding a separate encapsulation around each of an array of LEDs mounted directly to the board. Other features and advantages of the invention will become apparent from the following description and from the claims.